The invention relates to devices for fixation of the bone ends at each side of a fracture.
It is known that the healing of bone fractures may be controlled when the bone ends at each side of the fracture is fixated adjacent to each other. Even if a part of the bone is missing the ends of the bones may during the healing be drawn slowly apart and the increased space between the bone ends will then be filled up with new bone mass.
It is important that the bone is not substantially loaded until the healing process has developed sufficiently to carry the load and consequently a method and an apparatus enabling a measuring of the healing status of the bone without loading it excessively is wished for. However the strength of the healed bone may only be measured by loading the bone moderately and measuring its response to the load.
U.S. Pat. No. 5,339,533 describes a system for measuring the stiffness of a healing fractured bone. By this system the fixation device is dismounted and replaced by a bending stress goniometer. With one end of the fractured bone resting on a load cell the fractured part is pressed until a bending of 0,5-1.degree. is obtained and the load on the load cell is recorded. After the measuring the fixation rod is remounted unless the measuring shows that the bone has obtained sufficient strength so that further treatment can be stopped. A repetitive dismounting and remounting of the fixture means risk of dislocation of the bone ends when they are not supported by an external fixture and during the remounting.
From EP 0 117 859 a method for indirectly measuring the strength of a healing bone is described. This is obtained by equipping a clamping bar connecting pins fixed in the bone ends at each side of the fracture with strain gauge measuring strips by which it may continuously be measured which load is carried by the external fixture. By this method the external fixture is not removed and it may be estimated to which extent a load is carried by the fixture, but an explicit value reflecting the strength of the healing bone is not obtained.
EP-A-0324279 describes an apparatus for measuring fracture stiffness. The apparatus comprises a goniometer which is positionable to run substantially parallel to the long axis of the bone having the fracture and which has one end connected with a first spacer, placed axially to one side of the fracture, and the other end connected with a second spacer place axially to the other side of the fracture, and said apparatus including means to measure the signal of the goniometer to obtain a measurement of angular deformation on the application of a load, said measurement being representative of fracture stiffness. By this apparatus the bending of the bone is measured as a function of the load applied. Care is taken to avoid errors due to rotation at the fracture.